The present invention relates to an improvement in a gasket-sealed casing for a fixed disc memory unit or, more particularly, to an improvement in a casing for a fixed disc memory unit consisting of a case body and a covering member jointly forming the casing with an air-tight sealing of a gasket along the peripheral flange between the case body and the covering member, whereby the casing is imparted with greatly improved reliableness and stability of the air-tight sealing.
As is well known, many of conventional computers and other data processing electronic instruments utilize a floppy disc as an external data storage medium. It is a trend in recent years that the floppy discs are increasingly being replaced with fixed disc memory units by virtue of the much larger capacity of memory storage thereof than floppy discs and the much shorter access time for reading-out of the stored information. The above mentioned fixed disc memory unit is a thin rigid disc usually made of an aluminum alloy and coated with a layer of a material for magnetic recording, which is encased in a casing consisting of a case body and a covering member jointly forming the casing with a rubbery gasket therebetween to ensure air-tightness. The disc member is rotated at a high speed by means of a spindle motor while a magnetic head to face the rotating disc keeping a very narrow clearance therebetween of only about 0.5 .mu.mm serves to write-in and read-out the magnetically recorded information into and out of the disc.
As is readily understood from the above described working principle of a fixed disc memory unit, very serious troubles are caused by dusts entering the clearance between the rotating disc and the magnetic head. Accordingly, it is usual that such a fixed disc is encased together with a spindle motor in an air-tightly sealed casing consisting jointly of a case body and a covering member with a rubbery gasket member along the peripheral flange at which the two parts are jointed together. The air inside of the casing should be kept always as clean or dust-free as possible by means of high-performance filters for internal air circulation and for ventilation with the outer atmosphere. In this regard, the gasket member is required to be extremely reliable and stable to ensure air-tightness over the life of the memory unit. Needless to say, it is essential that no dust is produced from the-gasket member per se.
Various rubbery materials, both in a spongy form or in a solid, i.e. non-spongy, form, have been proposed as a material of the gasket member including silicone-based, i.e. organopolysiloxane-based, rubbers, urethane rubbers, polychloroprene rubbers, NBR rubbers and the like as well as certain thermoplastic resins having rubbery elasticity such as polyethylenes. These rubbery materials are molded and cured into a sheet having a specified thickness which is punched by using a punching die into the form of a desired gasket member or, alternatively, molded and cured in the form of the desired gasket member using a metal mold.
It is a conventional procedure that, in order to facilitate mounting of the above mentioned preshaped gasket member on the peripheral flange of the case body or the covering member, the preshaped gasket member is coated with a sticky adhesive with a release paper wrapping the adhesive-coated surface for temporary protection, which is removed prior to mounting of the gasket member on the peripheral flange. Alternatively, the peripheral flange is coated beforehand with a sticky adhesive before the gasket member is mounted thereon. These methods using a sticky adhesive are not always quite satisfactory as an industrial process because of the low working efficiency therefor in addition to possible drawbacks that the adhesive-coated surface is highly susceptible to dust deposition and a difficulty is encountered in the exact positioning to mount the gasket member on the correct position consequently leading to an increase in the manufacturing cost of the disc unit.
It should be noted, moreover, that preparation of die-punched gasket members is economically disadvantageous in respect of the costs for the punching dies and occurrence of a large amount of punching debris which must be disposed with some additional costs in order not to cause a problem of environmental pollution. Preparation of molded gasket members is also economically disadvantageous in respect of the costs for the metal molds in addition to a possible problem that the surface finishing of molded gaskets is sometimes incomplete eventually leading to formation of dust particles thereby.
One of the inventors and his co-inventor have recently proposed, in U.S. Pat. No. 4,950,521, in situ formation of a rubbery gasket member on the peripheral flange of the covering member of such a casing by the so-called formed-in-place gasket (FIPG) method in which a flowable and room temperature-curable or heat-curable silicone rubber composition is put as extruded from a dispenser nozzle in the form of a gasket along the peripheral flange followed by curing in situ to form a cured gasket member. This method is indeed effective to solve almost all of the problems m the above described prior art gasket members but has another problem that silicone rubber gasket thus formed has a relatively high coefficient of moisture permeation of as large as 50 to 100 g.multidot.mm/m.sup.2 .multidot.24 hours in an atmosphere of 90.+-.2% relative humidity at 40.+-.0.5.degree. C. so that the fixed disc memory unit assembled therewith can be used only under limited ambient conditions in order to avoid possible troubles caused by moisture.